Summary

The respiratory and photosynthetic electron transport chains of the two facultative phototrophsRhodobacter(Rb.) sphaeroides andRb.Capsulatus are arranged in such a way to be spatially segregated in separate regions of the internal membrane system (CM and ICM). The CM part contains the majority of the oxidative redox components which are therefore in redox non-equilibrium with most of the photochemical RCs; conversely, the major part of the photosynthetic carriers (including RCs, Cytc2 Cytcy and Cytbc1 complex) are located in the ICM part of the membrane. This spatial level of organization is paralleled by an arrangement of these photosynthetic elements in supramolecular complexes in order to allow a fast and efficient cyclic electron transfer by limiting the diffusion of the reactants. However, these two levels of arrangement are not present in all types of photosynthetic bacteria. Indeed, species likeBlastochloris viridis orRubrivivax gelatinosus, contain a large excess of RCs over the Cytbc1 complexes so that the formation of superscomplexes is stoichiometrically hindered. Further, in obligate aerobic phototrophs such as for example Rosebacter denitirficans, the Qa is fully reduced under anaerobic conditions and this might be due to the lack of both quinol oxidase and ICM system.

The expression of photosynthetic and respiratory components is controlled by the oxygen tension and by the redox state of the system. This genetic coordination mechanism does not necessary require a direct interaction of the two sets of components in line with their different spatial membrane location. The signals to which the system responds originate from either specific respiratory components, e.g.cbb3 oxidase, as in the case of oxygen sensing, or from redox carriers involved in both oxidative and photosynthetic ET, as for redox sensing. Although the genetic control of the supramolecular arrangement of the ETCs is, at present largely undefined, the working scheme presented here, suggests a tentative framework of genetic regulatory connections inRb.capsulatus and/orRb.sphaeroides